The aim of this grant proposal is to genetically and molecularly characterize vegetative incompatibility in the filamentous fungus, Neurospora crass. Vegetative incompatibility is a type of programmed cell death that can prevent transfer of infectious agents such as mycoviruses, throughout a fungal population. Vegetative incompatibility is universal in filamentous fungi, but is apparently absent from S. cerevisiae and S. pombe, presumably because of their unicellular or pseudohyphal growth habit. Unlike other filamentous fungi, N. crassa has a genetically well-defined vegetative incompatibility system (het loci) and both genetic and molecular tools are available. We have previously cloned and characterized three het loci, mating-type, het-c and het-6. This grant proposal describes experiments to elucidate the mechanism of vegetative incompatibility mediated by het-c in N. crassa. Our hypothesis concerning the basis of het-c mediated vegetative incompatibility states that the co-localization of alternative het-c polypeptides in a common cytoplasm triggers death by interference with the regulation of mitosis. We believe that an analysis of vegetative incompatibility in N. crassa will reveal links between cell cycle regulation and induction into programmed cell death in a simple eukaryote. The Specific Aims of this grant proposal are 1. To delineate the requirements for het-c allelic specificity, 2. Localize HET-C within the cell during growth and during programmed cell death, 3. To dissect the mechanism of vegetative incompatibility using both biochemical and genetic analyses. The analysis of vegetative incompatibility using both biochemical and genetic analyses. The analysis of vegetative incompatibility in filamentous fungi will provide information on the evolution of programmed cell death and may uncover mechanisms of cell cycle regulation and cell death that are not accessible by other means. Understanding the pathway of programmed cell death in filamentous fungi may provide unique targets for the development of fungal-specific drugs to combat human diseases caused by filamentous fungi.